Hi Julie....we did this run last Sept/Oct.
bmi transatlantic comes highly recommended if you read any of the frequent flyer sites. We originated in Glasgow so can't comment on MAN check in.
The planes are fairly new Airbus A330. In economy the seat pitch is a pretty decent 32", 38" in New Economy and recline to 160 degrees in The Biz. The same seats are used in New Economy and in Economy although the seat pitch is different. Very comfortable. Fortunately, we were upgraded to New Economy both ways.
Food is very good, for airline food. There is a choice of 3 entrees for the main meal, including a vegetarian option. You can have as much booze as you want....nowadays I cross the pond TT as I drive from MCO.
IFE....excellent. Choice of a dozen or so movies (unfortunately not screened on demand), plus various TV channels (comedy, kids, etc), and many Nintendo games.
At IAD you clear immigration and customs....collect bags and re check them straight afterwards. Then go upstairs and find your gate. Fortunately the arrival and departure flights use the same Concourse, but you may be in for a long walk as its very big. Obviously at MCO you're all set for a quick getaway.
Even though we flew on 3 planes each way, the journey was nicely broken up. It certainly was not at all tiring. I would happily go that route again....may even do so next year after we've tested VS (MAN - MCO) this summer.